1. Field of the Invention
The present invention relates to an optical semiconductor element package and to a manufacturing method thereof.
2. Description of the Background
A conventional optical semiconductor element package is composed of a metal bottom plate 11 and a metal frame 13 fixed to the top of the metal bottom plate to form a housing as shown in the perspective view of FIG. 4 and the plan view of FIG. 5. Optical semiconductor elements such as a semiconductor laser element and Peltier element for cooling the element are mounted on the metal bottom plate 11.
Generally speaking, heat generated from the optical semiconductor elements is cooled by the Peltier element underlying the optical semiconductor elements and exhausted to the outside of a module from a side opposite to the Peltier element through the metal bottom plate 11 in the end.
A window frame 14 to which an optical fiber is attached is installed in the metal frame 13 and an electric signal input and output terminal 12 is attached to each side portion of the metal frame 13. Pads 16 to be bonded to external leads 15 are provided on the outer side of the terminal 12 and pads 17 to be wire bonded to the optical semiconductor elements etc. arranged in the inside of the housing are provided on the inner side of the terminal 12.
In the optical semiconductor element package thus constituted, when the distance of each of bonding wires for connecting the optical semiconductor elements to the pads 17 is too long, inconvenience occurs by loose wires or the like. Therefore, the pads 17 internal to the housing are collected around the semiconductor elements unlike the pads 16 external to the housing.
That is, the pads 17 internal to the housing also function to convert the pitches of the wires. They also function to avoid cross wiring because a short circuit may occur when the bonding wires cross one another.
When it is difficult to provide all the wires on the surface layer portion of a wiring board, a multi-layer substrate is often used for the wiring board (terminal) 12. A metal cover for sealing the housing air-tightly is placed on top of the metal frame 13.
The metal frame 13 is desirably made of a material having a thermal expansion coefficient close to that of the electric signal input and output terminal 12 that is mainly made of a ceramic. An Fexe2x80x94Ni alloy or Fexe2x80x94Nixe2x80x94Co alloy is used as the material.
The electric signal input and output terminal 12 mainly made of ceramic is bonded to the metal frame by brazing or the like. Therefore, as shown in FIG. 6, a bonding portion having four faces (a face A, two faces C and a face D) is metallized. Stated more specifically, pad faces B that become electrodes and the upper and lower faces A and D of the terminal are metallized with W or the like, the bonding portion is cut to a predetermined length, and the cut faces C are metallized with Moxe2x80x94Mn or the like in most cases.
The metal bottom plate 11 must be made of a material having high heat conductivity because it must have a thermal expansion coefficient close to that of the metal plate 13 and quickly radiate heat generated from the semiconductor laser elements or the like. A Cuxe2x80x94W alloy etc. is used as the material. The metal frame 13 and the metal bottom plate 11 are bonded together by solder.
In the conventional optical semiconductor element package shown in FIG. 4 and FIG. 5 which has been described above, the electric signal input and output terminals 12 which are mainly made of ceramic such as alumina must be attached to the metal frame 13 to keep airtightness. Therefore, it must be mated with a recessed attachment portion formed in the metal frame 13 with an extremely strict dimensional tolerance. However, it is not easy to machine the terminal 12 mainly made of ceramic with an extremely strict dimensional tolerance after the above first step, thereby greatly boosting costs.
The two-stage metallization of four faces (a face A, two faces C and a face D) other than the pad faces as described above costs dear.
To eliminate the above problems, it is conceivable that the wiring board for inputting and outputting electric signals is arranged in the inside of the metal frame 13, the external leads 15 are drawn to the outside through through holes 18 formed in the side portion of the metal frame 13, and the through holes 18 are sealed up with a sealing material such as a glass material.
To this end, bonding portions between the wiring board and the external leads 15, which are external to the metal frame 13, must be incorporated in the metal frame 13. However, as an optical semiconductor module must be small in size, it is impossible to use a metal frame 13 at least larger than a conventional optical semiconductor element package so as to increase the internal space of the metal frame 13.
Meanwhile, to radiate heat generated from the optical semiconductor elements without fail, it is necessary to reduce heat resistance as much as possible between the optical semiconductor elements and the Peltier element and between the Peltier element and the metal bottom plate 11, which form a heat radiation route. A wiring board having low heat conductivity cannot be interposed between them. Further, an optical system for transmitting light emitted from the optical semiconductor elements to an optical fiber without fail must be maintained.
It is also conceivable that the wiring board is eliminated and the external leads 15 and the optical semiconductor elements or the like are directly wire bonded together. However, since the pitch of the external leads 15 is not changed in this case, bonding length becomes larger and inconvenience such as a short circuit caused by loose wires easily occurs.
Therefore, it is necessary to incorporate a wiring board for changing the pitch in the metal frame 13. However, no optical semiconductor element package that satisfies the above restraints and obtains the above effects is yet existent.
In view of the above problems, it is an object of the present invention to provide an inexpensive optical semiconductor element package which has excellent heat radiation properties and prevents a short circuit caused by loose wires.
It is another object of the present invention to provide a method of manufacturing the above optical semiconductor element package.
According to the present invention, there is provided an optical semiconductor element package including: a housing including a metal frame and a metal bottom plate, for storing optical semiconductor elements; electric signal input and output wiring boards arranged in the housing at positions so that the optical semiconductor elements are not existent right above and right below the boards; and external leads drawn to the outside through the side wall of the metal frame, wherein the wiring boards are connected to the external leads and to the optical semiconductor elements by bonding wires, and the input and output of an electric signal between the outside and the optical semiconductor elements are carried out through the bonding wires, the wiring boards and the external leads.
In this case, the wiring boards can be formed of a glass ceramic material.
In the above optical semiconductor element package, the wiring boards can have upper electrodes to be wire bonded to the optical semiconductor elements on the top surface and lower electrodes to be bonded to the external leads on the under surface, and spacers can be arranged between the metal bottom plate of the housing and the wiring boards.
The spacers can be formed as part of the metal frame or the metal bottom plate. Further, the external leads are drawn to the outside through holes formed in the side portion of the metal frame, and portions around the external leads in the through holes can be sealed up with a sealing material such as a glass material.
According to the present invention, there is also provided a method of manufacturing the above optical semiconductor element package, wherein the spacers are formed integrally with the metal frame or the metal bottom plate by metal injection molding.